1. Field of the Invention
The present invention relates to a modulation apparatus for generating a wide-band frequency modulated signal (hereinafter referred to as FM signal) through an optical frequency modulation scheme of a semiconductor laser and an optical heterodyne detection technique.
2. Description of the Background Art
FIG. 14 is a block diagram showing the configuration of a conventional modulation apparatus. Operation of such frequency modulation apparatus is described in detail in documents such as xe2x80x9cOptical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systemsxe2x80x9d, K. Kikushima, et al, IOOC 1995 Technical Digest, Vol. 5 PD2-7, pp. 33-34, which is incorporated herein by reference. The frequency modulation apparatus shown in FIG. 14 includes a signal source 700, an optical modulator 702, a local light source 703, a first optical waveguide 706, a second optical waveguide 708, and an optical receiver 714.
In the above frequency modulation apparatus, the signal source 700 produces an electrical signal Si, which is an original signal for FM modulation. The optical modulator 702 is constructed of, for example, a semiconductor laser. In general, the semiconductor laser emits light having a constant optical frequency f1, provided that an injection current is constant. When the injection current is amplitude-modulated, the optical frequency is also subjected to modulation, and the semiconductor laser emits an optical-frequency-modulated signal centering on the optical frequency f1. With this characteristic, the optical modulator 702 converts the electrical signal Si supplied from the signal source 700 into an optical-frequency-modulated signal L1. The local light source 703 produces an unmodulated light L2 having a constant optical frequency f2. The lights L1 and L2 are supplied to the optical receiver 714 through the first and second optical waveguides 706 and 708, respectively. The optical receiver 714 is constructed of a photodiode having square-law detection characteristics, for example, producing a beat signal of the supplied two lights L1 and L2 at a frequency fs equal to the optical frequency difference |f1xe2x88x92f2| between the two optical signals L1 and L2 (this operation is called optical heterodyne detection). The beat signal obtained in the above described manner is outputted as a frequency-modulated signal Sfm whose original signal is the electrical signal Si from the signal source 700.
As described above, with the use of high frequency modulation efficiency of the semiconductor laser (more than ten times the frequency modulation efficiency in ordinary electric circuit systems), the conventional frequency modulation apparatus shown in FIG. 14 can easily generate an extremely high-frequency, wide-band FM signal with large frequency deviation, which is difficult to be produced in the ordinary electric circuit.
However, light sources such as semiconductor lasers generally have large phase noise, compared with electric oscillators. Therefore, the above conventional modulation apparatus has a unique problem that white noise components increase at demodulation of the FM signal. More specifically, when the optical-frequency-modulated signal L1 from the optical modulator 702 and the unmodulated light L2 from the local light source 703 have frequency spectrums as shown in FIG. 15A, the frequency spectrum of the FM signal Sfm outputted from the optical receiver 714 becomes as such shown in FIG. 15B. As shown in FIGS. 15A and 15B, the phase noise included in the FM signal Sfm becomes the sum of the phase noises included in the optical-frequency-modulated signal L1 and the unmodulated light L2. Therefore, the white noise components increase when the FM signal is demodulated.
Therefore, an object of the present invention is to provide a frequency modulation apparatus capable of realizing high-frequency, wide-band frequency modulation and suppressing phase noise included in the frequency-modulated signal, with a combination of optical frequency modulation of a semiconductor laser and optical heterodyne detection.
The present invention has the following features to achieve the object above.
A first aspect of the present invention is directed to a frequency modulation apparatus for converting an input electrical signal into an FM signal through optical frequency modulation and optical heterodyne detection using a first light source emitting first light and a second light source emitting second light, the first light and the second light having different optical frequencies from each other; the apparatus comprising:
a first optical modulator for outputting the first light frequency-modulated with the input electrical signal as a first optical signal;
a beat signal generating part for generating an unmodulated beat signal corresponding to a carrier component of a beat signal obtained from the first and second lights through optical detection based on a square-law detection characteristic;
a frequency converter for generating the frequency-converted signal by converting frequency of the unmodulated beat signal;
a second optical modulator for generating a second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first optical signal and the second light with the frequency-converted signal; and
an optical receiver for receiving one of the first optical signal and the second light, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, and generating the FM signal through optical detection based on the square-law detection characteristic.
In the first aspect, two light sources (the first and second light sources) and two square-law detectors (the beat signal generating part and the optical receiver) constitute two optical heterodyne systems. A first optical heterodyne system is constructed of the first optical modulator corresponding to the first light source, the second light source as a local light source, and the beat signal generating part. A second optical heterodyne system is constructed of the first optical modulator, the second light source (local light source), and the optical receiver. With the frequency-converted signal obtained by converting the frequency of the carrier component (center frequency component) of the beat signal generated in the first optical heterodyne system, one of the two lights in the second optical heterodyne system is amplitude-modulated or intensity-modulated. Thus, the frequency modulation apparatus of the first aspect can suppress the phase noise of the FM signal, which is a beat signal generated in the second optical heterodyne system, realizing frequency modulation with low noise.
According to a second aspect, in the first aspect,
the first optical modulator generates the first optical signal through direct modulation; and
the beat signal generating part comprises
a photo-detection device for receiving the first optical signal and the second light, and generating a modulated beat signal, which is a modulated electrical signal having a center frequency equal to a difference of optical frequencies between the first optical signal and the second light, through optical detection based on the square-law detection characteristic; and
a filter for extracting a carrier component from the modulated beat signal, and outputting the carrier component as the unmodulated beat signal.
In the second aspect, the first optical signal is generated through direct modulation. Then, an unmodulated beat signal, which is the carrier component, is extracted by the filter from the modulated beat signal generated from the first optical signal and second light. With the frequency-convert signal obtained by converting the frequency of the unmodulated beat signal, one of two lights in the second optical heterodyne system is amplitude-modulated or intensity-modulated. Thus, the frequency modulation apparatus of the second aspect can suppress the phase noise of the FM signal generated by the optical receiver, realizing frequency modulation with low noise.
According to a third aspect, in the first aspect,
the first optical modulator comprises
the first light source for emitting the first light as being unmodulated; and
an external optical modulator for generating the first optical signal by modulating the first light emitted from the first light source with the input electrical signal;
the second light source emits second light as being unmodulated; and
the beat signal generating part comprises a photo-detection device for receiving the first and second lights, and generating the unmodulated beat signal through optical detection based on the square-law detection characteristic.
In the third aspect, the first light is converted through external modulation into the first optical signal, which is a modulated light. In the beat signal generating part, an unmodulated beat signal is generated from the first and second lights, which are both unmodulated lights. Therefore, no filter for extracting a carrier component from the modulated beat signal is required.
According to a fourth aspect, in the first aspect,
the first optical modulator generates the first optical signal having a predetermined center frequency f1, by uniquely converting an amplitude variation of the input electrical signal into an optical frequency variation of the first light,
the second light source emits light having a predetermined frequency f2 as the second light,
the beat signal generating part comprises
a photo-detection device for receiving the first optical signal and the second light, and generating a modulated beat signal, which is a modulated electrical signal, at a frequency fs=|f1xe2x88x92f2 equal to a difference of optical frequencies between the first optical signal and the second light through the optical detection based on the square-law detection characteristic; and
a filter for extracting a carrier component from the modulated beat signal, and outputting the carrier component as the unmodulated beat signal,
the frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputting the converted signal as the frequency-converted signal,
the second optical modulator generates a second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first optical signal and the second light with the frequency-converted signal having the frequency fx, and
the optical receiver receives one of the first optical signal and the second light, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, generating the FM signal at a frequency fL=|fsxe2x88x92fx| through optical detection based on the square-law detection characteristic.
In the fourth aspect, the input electrical signal is converted into an optical-frequency-modulated signal by the first optical modulator. In the above-described first optical heterodyne system, the carrier component, which is the unmodulated beat signal outputted from the beat signal generating part, is frequency-converted. In the above-described second optical heterodyne system, the light signal from the first optical modulator or the light from the second light source is amplitude-modulated or intensity-modulated with the frequency-converted carrier component. Thus, the frequency modulation apparatus of the fourth aspect can suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation with low noise characteristics.
According to a fifth aspect, in the first aspect,
the frequency modulation apparatus further comprises
a branching circuit for branching the input electrical signal into first and second electrical signals with opposite phases; and
a third optical modulator for converting the second electrical signal into a third optical signal through optical intensity modulation,
the first optical modulator generates the first optical signal having a predetermined center frequency f1 by uniquely converting an amplitude variation of the first electrical signal into an optical frequency variation of the first light through direct modulation,
the second light source emits light having a predetermined frequency f2 as the second light,
the beat signal generating part comprises:
a photo-detection device for receiving the first optical signal and the second light, and generating a modulated beat signal, which is a modulated electrical signal, at a frequency fs=|f1xe2x88x92f2| equal to a difference of optical frequencies between the first optical signal and the second light through optical detection based on the square-law detection characteristic; and
a filter for extracting a carrier component from the modulated beat signal, and outputting the carrier component as the unmodulated beat signal,
the frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the signal as the frequency-converted signal,
the second optical modulator generates a second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first optical signal and the second light with the frequency-converted signal having the frequency fx, and
the optical receiver receives one of the first optical signal and the second light, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, generating the FM signal at a frequency fL=|fsxe2x88x92fx| through the optical detection based on the square-law detection characteristic, and also receives the third optical signal, generating an electrical signal corresponding to an optical-intensity-modulated component included in the third optical signal through the optical detection.
In the fifth aspect, the input electrical signal, which is an original signal for frequency modulation, is branched into first and second electrical signals of opposite phase. The first electrical signal is then converted by the first optical modulator into a first optical signal, which is an optical-frequency-modulated signal. In the above-described first optical heterodyne system, the carrier component, which is an unmodulated beat signal outputted from the beat signal generating part, is frequency-converted. In the above-described second optical heterodyne system, the optical signal from the first optical modulator or the light from the second light source is amplitude-modulated or intensity-modulated with the frequency-converted carrier component. Furthermore, the second electrical signal is converted by the third optical modulator into a third optical signal, which is an optical-intensity-modulated signal, and then supplied to the optical receiver. In the optical receiver, an intensity-modulation-direct-detection component (IM-DD component) is generated through square-law detection. This IM-DD component cancels out an IM-DD component corresponding to an optical-intensity-modulated component (generated due to direct modulation) included in the first optical signal. Thus, the frequency modulation apparatus of the fifth aspect can suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation of high quality with undesired components reduced.
According to a sixth aspect, in the fifth aspect,
the frequency modulation apparatus further comprises a phase/amplitude adjusting part for adjusting a first IM-DD component and a second IM-DD component to have opposite phases and a same amplitude, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the second IM-DD component corresponding to an optical-intensity-modulated component included in the third optical signal, the first and second IM-DD components being generated by the optical receiver through optical detection based on the square-law detection characteristic.
In the sixth aspect, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the second IM-DD component corresponding to an optical-intensity-modulated component included in the third optical signal are adjusted to have opposite phases and the same amplitude. Thus, the frequency modulation apparatus of the sixth aspect can more surely suppress the IM-DD components outputted from the optical receiver, generating high-frequency, wide-band FM signal of higher quality.
According to a seventh aspect, in the first aspect,
the frequency modulation apparatus further comprises:
a branching circuit for branching the input electrical signal into the first and second electrical signals having opposite phases to each other; and
a fourth optical modulator for outputting the second light, which is a modulated light having a predetermined center frequency f2, as a fourth optical signal,
the first optical modulator generates the first optical signal having a predetermined center frequency f1 by uniquely converting an amplitude variation of the first electrical signal into an optical frequency variation of the first light through direct modulation,
the fourth optical modulator generates the fourth optical signal having the predetermined center frequency f2 by uniquely converting an amplitude variation of the second electrical signal into an optical frequency variation of the second light by direct modulation,
the beat signal generating part comprises
a photo-detection device for receiving the first and fourth optical signals, and generating a modulated electrical beat signal at a frequency fs=|f1xe2x88x92f2| equal to a difference of optical frequencies between the first and fourth optical signals through optical detection based on the square-law detection characteristic; and
a filter for extracting a carrier component from the modulated beat signal, and outputting the carrier component as the unmodulated beat signal,
the frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency-converted signal,
the second optical modulator generates the second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first and fourth optical signals with the frequency-converted signal, and
the optical receiver receives one of the first and fourth optical signals, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, generating the FM signal at a frequency fL=|fsxe2x88x92fx| through optical detection based on the square-law detection characteristic.
In the seventh aspect, the input electrical signal, which is an original signal for frequency modulation, is branched into first and second electrical signals of opposite phase. The first electrical signal is converted through direct modulation in the first optical modulator into a first optical signal, which is an optical-frequency-modulated signal. The second electrical signal is converted through direct modulation in the fourth optical modulator into a fourth optical signal, which is an optical-frequency-modulated signal. The present frequency modulation apparatus constitutes two optical heterodyne systems. A first optical heterodyne system is constructed of the first optical modulator, the fourth optical modulator, and the beat signal generating part. A second optical heterodyne system is constructed of the first optical modulator, the fourth optical modulator, and the optical receiver. In the first heterodyne system, the carrier component, which is an unmodulated beat signal outputted from the beat signal generating part, is frequency-converted. In the second heterodyne system, the optical signal outputted from the first or fourth optical modulator is amplitude-modulated or intensity-modulated with the frequency-converted carrier component. Thus, the frequency modulation apparatus of the seventh aspect can suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation with low noise. Furthermore, as described above, the frequency-modulated signal is generated by push-pull operation of the first and fourth optical modulator, canceling out an undesired IM-DD component caused due to optical-intensity-modulated component generated in direct modulation. This enables frequency modulation of high quality with undesired components reduced.
According to an eighth aspect, in the seventh aspect,
the frequency modulation apparatus further comprises a phase/amplitude adjusting part for adjusting a first IM-DD component and a third IM-DD component to have opposite phases and a same amplitude, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the third IM-DD component corresponding to an optical-intensity-modulated component included in the fourth optical signal, the first and third IM-DD components being generated by the optical receiver through optical detection based on the square-law detection characteristic.
In the eighth aspect, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the third IM-DD component corresponding to an optical-intensity-modulated component included in the fourth optical signal are adjusted to have opposite phases and a same amplitude. Thus, the frequency modulation apparatus of the eighth aspect can more surely suppress the IM-DD components outputted from the optical receiver, generating high-frequency, wide-band FM signal of higher quality.
According to a ninth aspect, in the first aspect,
the frequency modulation apparatus further comprises an optical filter inserted between the first optical modulator and the beat signal generating part, the optical filter extracting an optical carrier component from the first optical signal,
the second light source emits unmodulated light as the second light, and
the beat signal generating part comprises a photo-detection device for receiving the optical carrier, component extracted by the optical filter and the second light, and generating the unmodulated beat signal through optical detection based on the square-law detection characteristic.
In the ninth aspect, the input electrical signal, is converted by the first optical modulator into a first optical signal, which is an optical-frequency-modulated signal. In the first optical heterodyne system as described in the first aspect, the optical carrier component is extracted from the first optical signal by the optical filter inserted between the first optical modulator and the beat signal generating part. The beat signal generating part generates an unmodulated beat signal from this optical carrier component and the second light without using a filter. This unmodulated beat signal, that is, the carrier component, is frequency-converted to be a frequency-converted signal. Then, in the second optical heterodyne system as described in the first aspect, the optical signal from the first optical modulator or the light from the second light source is amplitude-modulated or intensity-modulated with the above frequency-converted signal (frequency-converted carrier component). Thus, the frequency modulation apparatus of the ninth aspect can suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation with low noise.
According to a tenth aspect, in the first aspect,
the frequency modulation apparatus further comprises
a branching circuit for branching the input electrical signal into first and second electrical signals having opposite phases to each other;
a fourth optical modulator for outputting the second light, which is a modulated light having a predetermined center frequency f2, as a fourth optical signal;
a first optical filter inserted between the first optical modulator and the beat signal generating part; and
a second optical filter inserted between the fourth optical modulator and the beat signal generating part,
the first optical modulator generates the first optical signal having a predetermined center frequency f1 by uniquely converting an amplitude variation of the first electrical signal into an optical frequency variation of the first light through direct modulation,
the fourth optical modulator generates the fourth optical signal having the predetermined center frequency f2 by uniquely converting an amplitude variation of the second electrical signal into an optical frequency variation of the second light through direct modulation,
the first optical filter extracts an optical carrier component from the first optical signal,
the second optical filter extracts an optical carrier component from the fourth optical signal,
the beat signal generating part comprises a photo-detection device for receiving the optical carrier components extracted by the first and second optical filter, and generating the unmodulated beat signal at a frequency fs=|f1xe2x88x92f2| corresponding to a difference of optical frequencies between the optical carrier components through optical detection based on the square-law detection characteristic,
the frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency-converted signal,
the second optical modulator generates the second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first and fourth optical signals with the frequency-converted signal having the frequency fx, and
the optical receiver receives one of the first and fourth optical signals, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, generating the FM signal at a frequency fL=|fsxe2x88x92fx| through optical detection based on the square-law detection characteristic.
In the tenth aspect, the input electrical signal, which is an original signal for frequency modulation, is branched into first and second electrical signals with opposite phases to each other. The first electrical signal is converted by direct modulation of the first optical modulator into a first optical signal, which is an optical-frequency-modulated signal. The second electrical signal is converted by direct modulation of the fourth optical modulator into a fourth optical signal, which is an optical-frequency-modulated signal. The present frequency modulation apparatus constitutes two optical heterodyne systems. A first optical heterodyne system is constructed of the first optical modulator, the fourth optical modulator, and the beat signal generating part. A second optical heterodyne system is constructed of the first optical modulator, the fourth optical modulator, and the optical receiver. In the first optical heterodyne system, the optical carrier components are extracted from the first optical signal by the first optical filter and from the fourth optical signal by the second optical filter, respectively. The beat signal generating part generates an unmodulated beat signal from these optical carrier components without using a filter. This unmodulated beat signal, that is, the carrier component is frequency-converted to generate a frequency-converted signal. In the second optical heterodyne system, the optical signal from the first or fourth optical modulator is amplitude-modulated or intensity-modulated with the above frequency-converted signal (frequency-converted carrier component). Thus, the frequency modulation apparatus of the tenth aspect can suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation of high quality with undesired components reduced.
According to an eleventh aspect, in the tenth aspect;,
the frequency modulation apparatus further comprises a phase/amplitude adjusting part for adjusting a first IM-DD component and a third IM-DD component to have opposite phases and a same amplitude, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the third IM-DD component corresponding to an optical-intensity-modulated component included in the fourth optical signal, the first and third IM-DD components being generated by the optical receiver through optical detection based on the square-law detection characteristic.
In the eleventh aspect, like the eighth aspect, the first IM-DD component corresponding to an optical-intensity-modulated component included in the first optical signal and the third IM-DD component included in the fourth optical signal are adjusted to have opposite phases and a same amplitude. Thus, the frequency modulation apparatus of the eighth aspect can more surely suppress the IM-DD components outputted from the optical receiver, generating high-frequency, wide-band FM signal of higher quality.
According to a twelfth aspect, in the first aspect,
the frequency modulation apparatus further comprises:
a first propagation time adjusting part for equalizing an optical and electrical propagation time in a path from the first light source through the beat signal generating part to the optical receiver and an optical propagation time in a path from the first light source directly to the optical receiver; and
a second propagation time adjusting part for equalizing an optical and electrical propagation time in a path from the second light source through the beat signal generating part to the optical receiver and an optical propagation time in a path from the second light source directly to the optical receiver.
In the twelfth aspect, the optical signal outputted from the first light source is branched into two optical signals. The frequency modulation apparatus equally sets the propagation time of these two optical signals passing through each component or being subjected to optical-electrical or electrical-optical conversion until they reach the optical receiver. Also, the optical signal outputted from the second light source is branched into two optical signals. The frequency modulation apparatus also equally sets the propagation time of these two optical signal passing through each component or being subjected to optical-electrical or electrical-optical conversion until they reach the optical receiver. Thus, the frequency modulation apparatus of the twelfth aspect can appropriately suppress the phase noise of the FM signal, which is a beat signal outputted from the optical receiver, realizing frequency modulation with lower noise.